Against daunting obstacles, scientists race to make a vaccine as newly virulent malaria sweeps across the world, killing a million people a year.

Rip Ballou's world began to blur around the edges as he stood among the croquet wickets, sipping home-brewed beer at a friend's lawn party. He should have expected it. Two weeks earlier, he'd agreed to let infected mosquitoes land on his arm and fill their bellies with his blood.

The five other volunteers were already sick, but the 34-year-old physician still hoped he would be protected. Ballou and a team of Army scientists had devoted years to making a vaccine against malaria, and the young officer had eagerly volunteered to serve as one of the first guinea pigs.

Maybe it was the beer, he told his wife, as she drove them back to their Wheaton home that day in 1987. Maybe it was too much sun. Then a tremor shook his body, and he resigned himself to the truth. The vaccine had failed. Instead of defeating one of man's oldest foes, Ballou knew the organism had beaten him.

Malaria is a ferocious predator, each year claiming more than 1.1 million lives. In Africa, it kills enough children every day to fill 100 kindergarten classes. Once subdued in much of the world, malaria has rebounded and today infects up to a half-billion people a year. It has surfaced in recent years in southern Europe and Russia, where it was routed almost 40 years ago. As air travel draws the world closer together, experts increasingly fear malaria outbreaks in the United States.

Rich nations and the pharmaceutical industry pay little attention to the disease, which mainly strikes poor countries. But for nearly a century, a few scientists have dreamed of vanquishing malaria with a vaccine.

The more they discovered about the organism, though, the more it seemed to mock them. It was as if the parasite possessed a malignant intelligence, a deep understanding of the human body and its defenses.

Like the scientists before them, Ballou and his colleagues at the Walter Reed Army Institute of Research in Washington came to respect their slippery adversary. Sometimes, they doubted it could ever be conquered. For years, they struggled along on a threadbare budget, working in obscurity, enduring repeated frustrations.

Finally, last year, they could claim some success. For a short time, they protected hundreds of people from malaria in Africa, proving that a vaccine was at least possible. More work would be needed, they knew, more money and luck. But Ballou and his team had come closer than anyone before to creating the ultimate weapon against the disease.

After that first discouraging trial 13 years ago, the Walter Reed scientists learned to build on their failure. Back at the lab, they began asking themselves: What had gone wrong? Why had the vaccine failed? Most important, how could they beat malaria the next time?

One out of five

After the "Long Rains" raked the ragged fields along Kenya's coast, the fever returned to claim its melancholy harvest.

Ria Sulubu lay in a hospital ward in the town of Kilifi last summer, his soft brown skin shining against the white sheet. The 3-year-old panted. A desk lamp warmed him. His tiny fists clenched and unclenched, his eyes darting fitfully. Others in the cramped ward coughed or moaned.

In some villages in sub-Saharan Africa, malaria kills one out of five children before their fifth birthday. Unlike many other infectious diseases, malaria confers only limited immunity. Small children can contract malaria 30 or 40 times. Even if the disease doesn't kill them, it can cause severe brain damage or make them vulnerable to other infections. Some might be stricken with Burkitt's lymphoma, a cancer that causes balloon-shaped tumors on the face and abdomen.

Ria suffered from a severe complication called cerebral malaria. For 10 days, seizures jolted his brain. Scribbled in pencil, his medical chart for June 25 records more than a dozen episodes: 5:05 a.m., 6:10 a.m., 6:18 a.m., 6:36 a.m., 6:48 a.m. and so on. If doctors couldn't stop them, Ria would die.

Two weeks earlier, Ria had been bitten by a female mosquito, seeking the nourishment of human blood before she laid her eggs. Maybe Ria felt the insect puncture his skin as he played in the red earth outside his mother's mud-wattle house. Maybe the bite came while he was sleeping with his two sisters on their bed.

The insect's saliva carried Plasmodium falciparum, one of four species of the malaria parasite and the one responsible for almost all the deaths.

Eyelash-shaped parasites slipped through Ria's skin and into his blood. Within minutes, each parasite wiggled into a liver cell and adopted a beady shape, like a fish egg. They poked out a chemical net to snare nutrients from the blood.

Over the next 10 days, the parasites changed form, gorged and created tens of thousands of offspring. Leaving the liver, they invaded red blood cells, shifted shape again to fool the body's defenses, and multiplied. Every 48 hours, new parasites burst in unison from their host blood cells and swarmed into fresh cells.

With each eruption, the parasites created showers of cellular debris. That provoked Ria's immune system to pump out chemicals called cytokines, causing shattering chills, fevers and pain. Ria may have felt as though his muscles were pricked by needles, his head as though it were split in a vise.